Inkjet printers are widely used as low-cost printers. In an inkjet printer of this kind, rather than simply printing characters, it is necessary to print images. Therefore, tonal representation for each pixels and edge smoothing is required.
On the other hand, in laser printers, this can be achieved readily by varying the size of the dots, and altering the dot positions by pulse width modulation of the laser. However, in an inkjet printer, it is not easy to control the dot position or dot size at each nozzle. One of the reasons for this is that, whereas a laser printer performs all drawing operations by switching a single laser beam on and off, in an inkjet printer, many features of the printer depend on the particular head composition of the serial printer and the particular drive method of the inkjet printer, namely, that the nozzles are disposed in a vertical and horizontal lattice configuration, and that a common drive waveform is supplied to the drive elements driving each nozzle.
For the above reasons, it is difficult to perform control whereby the jet timing of a certain nozzle is shifted independently, and consequently, control of individual dot positions is difficult. In the case of a system for controlling the timing by providing independent drive sources for each nozzle, although such control is technically possible, given the fact that nozzles are currently increasing in number, this cannot be seen as a practicable system, from the viewpoints of circuit size or cost.
Furthermore, in an inkjet head using thermal elements, in order to reduce costs, time division matrix driving is implemented whereby the total group of nozzles are divided into a plurality of blocks, and each plurality of nozzles is driven simultaneously, and this means that it is just as difficult to shift the timing for one particular nozzle as it is with a piezoelectric system.
Therefore, in the prior art, proposals have been made for achieving tonal representation and smoothing in the case of inkjet heads.
The first such proposal is a method for changing the size of the recorded dot for one pixel by altering the amount of ink emitted, tonal representation being achieved by variation of the dot size, and smoothing being achieved by selecting the dot size (for example, Japanese Patent Laid-open No. H11-5298, Japanese Patent Laid-open No. H11-78005, and the like.)
The second represents each pixel as a plurality of dots of different diameters, and achieves tonal representation by varying the number of dots (for example, Japanese Patent Laid-open No. H11-115221, Japanese Patent Laid-open No. H10-81014, and the like).
However, the first drive method of the prior art requires a different drive waveform for each tone graduation, and hence it is difficult to achieve a low unit price. Moreover, although the size of the dots changes, the position remains the same, and therefore, whilst this is acceptable for tonal representation, it is not suitable for smoothing.
The second drive method of the prior art is able to control the number of dots per pixel, but it is essentially an extension of the first prior art method, and since it assumes a large number of tone graduations, a plurality of dots of different sizes are positioned within one pixels and hence the method is suitable for tonal representation, but it is not suitable for smoothing. And furthermore, similarly to the first prior art method, it requires a large number of different drive waveforms, which makes it difficult to achieve a low unit price.